Slightly alter the handling of LOCAL_LIB_DIRS to skip addition of
directories in LOCAL_LIB_DIRS if they are subdirectories of directories
listed in LOCAL_DIRS. This allows a hierarchy like:

foo
foo/lib
foo/usr.bin
foo/usr.sbin

to be supported with LOCAL_DIRS=foo LOCAL_DIRS=foo/lib.

MFC after: 1 week
Sponsored by: DARPA, AFRL